1. Field of the Invention
The present invention relates to a bank angle detecting device for a vehicle such as, for example, a motorcycle or a small sized planing boat, which device can accurately detect the vehicle bank angle during the cornering of the vehicle.
2. Description of the Related Art
When a motorcycle then travelling on the road surface approaches a corner, the rider undergoes a cornering with the motorcycle banked at a certain angle. The standard motorcycle currently commercially available in the market has a headlamp fixed to the motorcycle frame structure and, therefore, when the motorcycle is banked, the optical axis of the headlamp tilts in concomitance with the tilt or banking of the motorcycle. Thus, rays of light projected from the headlight towards a zone forwardly of the motor cycle and conforming to the eyesight of the rider during the cornering in the night tend to be reduced, resulting in narrowing the field of view forwardly of the travelling direction of the motorcycle.
In other words, as shown in FIG. 14 illustrating the forward sight as viewed by the motorcycle rider, when and so long as the motorcycle travels straightforward, the region of illumination (light distribution) A cast by the headlamp represents a region spreading leftwards and rightwards in a direction parallel to the horizontal line H, and in contrast, when the motorcycle turns in a leftward direction, shown by the arrow P, along a curved lane 90 as shown in, for example, FIG. 15 and does hence undergo a cornering with the body of the motorcycle banked leftwards, the region of illumination A cast by the headlamp tilts downwardly leftwards as compared with that assumed during the straight forward travel. As a result, the region of illumination A of the headlamp is reduced at an area encompassed within the forward sight of the motorcycle rider and positioned inwardly of the direction of turn (as indicated by a portion B enclosed within the phantom circle in FIG. 15), that is, forwards with respect to the direction of travel of the motorcycle then undergoing the cornering and, as a matter of fact, the rider's field of view forwardly of the direction of cornering is narrowed.
As a headlamp device designed to solve those problems discussed above, the JP Laid-open Patent Publication No. 2004-155404 discloses the headlight device in which based on the bank angle of the motorcycle body detected by a bank angle detector, a lens and a light emitting element of the headlamp are rotated in a direction reverse to the direction in which the motorcycle body is banked.
According to the headlamp device disclosed in the above mentioned patent publication, a yaw rate sensor such as a gyro is employed as the bank angle detector and the bank angle δ can be given by the following formula:δ=sin−1(v·R/g)  (1)wherein v represents the motorcycle speed, g represents the gravitational acceleration and R represents the yaw rate.
Thus, the headlamp is rotated an angle, corresponding to the bank angle δ so determined, in a direction reverse to the bank angle δ to secure the region of illumination of the headlamp that provides a large field of view.
However, the value R of the yaw rate sensor, employed in the above formula (1), is the sum of the actual yaw rate value plus the amount of sensor drift. The sensor drift amount referred to above is the sum of a zero point error, generated due to electric characteristic at the time the electric power is turned on, and the amount of time dependent change of the zero point due to an external factor such as, for example, change in temperature. If the sensor drift amount is zero, the bank angle δ of the motorcycle can be accurately calculated by means of the above formula (1). However, since the sensor drift amount is in practice not zero, under the influence of such a sensor drift the bank angle δ involves a probable error. This probable error is generally proportional to the motorcycle speed and the sensor drift amount and, therefore, particularly where the motorcycle speed is high, the probable error brought about by the sensor drift amount becomes large.